MOSFETs (active rectification) in place of diodes in linear PSU - diyAudio
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Old 11th January 2006, 10:50 AM   #1
UnixMan is offline UnixMan  Europe
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Question MOSFETs (active rectification) in place of diodes in linear PSU

Hi everybody,

I was wandering about an idea. Among tube lovers, it is widely accepted that in audio gears tube rectifiers generally gives better results (better sound) than sand state rectifiers, even when coupled with massive CLC filtering.

Indeed, there are some valid technical reasons behind this. Tube rectifiers are not affected by the (direct and reverse) switching noise, plus their transfer characteristic is such as to drastically reduce the rectification noise magnitude and spectral bandwidth thanks to the "softer" and "smoother" currents that flows into the filter capacitors.

Unfortunately, tube rectifiers have all sorts of well known drawbacks, too.

So I was wondering: why not trying to "emulate" the behaviour of tube rectifiers replacing the simple diodes with some active solid state circuitry?

Thinking about it, it became obvious that using some active circuitry it should be possible to hunt for (and achieve) even better results than the mere "emulation" of a tube rectifier. Better results which of course can include some degree of embedded voltage (pre-)regulation, too.

Possibly (but not necessarily, given the different goals) this can be sort of like the synchronous rectifiers used in some SMPS.

Then something came to my mind... I remember a long time ago I have read somewhere about someone who have written an article on some magazine (maybe Glass Audio, or MJ, or some other one... or was it the JAES? can't remember) proposing to replace diodes with MOSFETs in audio PSUs. That is, possibly something similar to what I was wondering about (don't really know, I never got to read the article itself, only heard about it).

Now the questions for you guys are:

what do you think about the idea?

did anybody tried to do something like that?

does anybody know about the aforementioned article?

Thanks a lot!
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Old 11th January 2006, 11:59 AM   #2
poobah is offline poobah  United States
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Seems like there would be some painless ways of doing this with special taps, "extensions" more precisely, of the power trans secondary... something on the order of 10 Volts higher (per leg) to drive the gates. The trick being that you would have to avoid waking up the body diode of the FET.

You would need a clear picture of the conducting angle to make sure you had the right amount of gate drive over the range of input voltages and load currents.

I 'spose you could test this easily enough with a second trans with a 1.1 step up ratio...or so. Actually, you could phase 2 heater windings on the secondary to pull this off... just for testing.
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Old 11th January 2006, 01:14 PM   #3
Eva is offline Eva  Spain
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Put a 10 to 100 ohm resistor in series with a 1N4007 diode and you'll have a nice tube rectifier.

Active rectification with MOSFETs is not an "improvement" because the body diode of the MOSFET is used, and it's much like a 1N4007 or a bit faster.

Also, diode behaviour is strongly mistified (usually by people that talks a lot about vague things like "noise" and "switching" but has never looked at the real thing with an oscilloscope). There is a thread showing actual current and voltage waveforms that I think it's worth reading :

Fast Recovery rectifier diodes

(The most interesting stuff starts at page 5 or 6).
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Old 11th January 2006, 01:50 PM   #4
poobah is offline poobah  United States
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These people need a P-channel tube... super-cooled anode surrounded by holes! -
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Old 11th January 2006, 01:57 PM   #5
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Quote:
Put a 10 to 100 ohm resistor in series with a 1N4007 diode and you'll have a nice tube rectifier.
Yes. I've done precisely that and when spectra were taken, the 1N4007 with series resistor showed less noise on the rail than either a GZ34 or a diode-connected 572B when the resistor was adjusted to give an equal voltage output.
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Old 11th January 2006, 02:58 PM   #6
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Eva, you can do some tricks with Infinoen's BSP, BST IC's. One trick is to connect a powerswitch backwards and let the internal charge pumps make the mosfet conducting with the wrong polarity.

I'm not sure this will work as rectifier but it sure works as a super diode with no losses! We use it commercial products with good results.

This is a real smart idea I would never thought of myself

Extremely smart reverse polarity protection
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Old 11th January 2006, 04:11 PM   #7
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Hi poobah,

Quote:
Seems like there would be some painless ways of doing this with special taps, "extensions" more precisely, of the power trans secondary... something on the order of 10 Volts higher (per leg) to drive the gates. The trick being that you would
mmmh... not sure if I got it right (could you be so kind to post some rough sketch?).

BTW, if I understood it correctly it looks a lot like (it is?) a self-driven synchronous rectifier... and indeed it sounds like an interesting idea to start with!

(I guess you'd need a center-tapped secondary... or do u think this can be arranged as a bridge, too?)

But then... there would be any real advantage here? If I understood it correctly, the MOSFETs are operated as switches, thus I guess that currents (and noise) will not be much different from what you would get from a conventional (diode) rectifier.

As I said, my goals are pretty different from what have pushed synchronous rectifiers into the SMPS world. I couldn't care less about efficiency (which is what SR are all about).

I do care mostly about reduction of rectification noise, and would welcome some degree of voltage regulation at the rectifier level (so that the following massive filtering will decouple it as much as possible from the signal path).

To fully implement my idea, the active devices (be that MOSFETs, BJTs or whatever happen to be the most appropriate ones for the task) should be operated in their linear region (dissipating fairly amounts of power) at least in some part of the cycle.

In the device control network there must be some R-C to limit the current slew-rate (that is, to limit the bandwith of the current pulses), as well as some voltage reference (and comparators, etc) to gradually reduce and eventually cut-off the charging current when the voltage across the filter cap approaches the set value (which of course must be fairly less than what you would get from a "normal" rectifier).

This way I _hope_ to get something /better/ than a silicon diode with a series resistor!

...which BTW will work quite well if you can afford to have a high enough R value in the circuit; I did that using an integrated bridge -> _470R_ -> 100uF (-> 30H -> 330uF) on some 380V/20mA PSU for the pre/driver section of my last "baby" (a 6C33 SET), and it is as noiseless as it could be... but try doing the same on the 200V/500mA+ PSU for the output tubes!

With this respect, I have to admit I do care a bit about efficiency... it should not end up producing more heat than the room heating system!
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Old 11th January 2006, 04:30 PM   #8
poobah is offline poobah  United States
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Paulo,

I don't know if I would bother with all this work for rectification. See what SY & EVA say above. Regular diodes can work well. It is a curious idea. Also, I haven't totally thought this thru... just a crude idea and a VERY crude drawing!

I think this would work in bridge or center tapped. Play with the idea... I would use sand...
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Old 11th January 2006, 07:19 PM   #9
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Quote:
Originally posted by SY


Yes. I've done precisely that and when spectra were taken, the 1N4007 with series resistor showed less noise on the rail than either a GZ34 or a diode-connected 572B when the resistor was adjusted to give an equal voltage output.
sure... see my previous post.

Problem is, with a fixed resistance in series with a diode you will not only limit the slew-rate of the pulse, but the max peak current as well. To be effective, this trick require to waste a sensible fraction of the power on the resistor, and will soon become impractical as the required PSU output current increase.

The active rectifier/regulator I'm proposing would dynamically adjust its series impedance during the cycle to limit the charging current slew-rate without necessarily limiting the max peak current.
On average (I hope!) it should be a bit more power efficient (though possibly a bit more noisy) than an "equivalent" D+R.

And for sure it would be much more efficient than a D+R followed by a conventional linear voltage regulator... in fact, I guess that (being partly "switching" by its very nature) it should also be more efficient than any conventional linear voltage regulator alone.

Of course, if it's followed by any serious (CLC and/or CRC) filter, it will provide no regulation against fast load fluctuations (well, unless some more tricks are used to also sense the overall PSU output voltage - that is, a feedback loop taken from the last capacitor of the chain...).

But this is a very desired feature indeed! I personally believe that any voltage regulator directly interacting with the audio circuit destroys the sound quality.


Quote:
Originally posted by poobah

I don't know if I would bother with all this work for rectification. See what SY &
see above... it would not be just for rectification!

Moreover I have to say that when it comes to hi-quality audio, all of the PSU -including rectifiers- deserve the maximum attention.

Having done a few experiments with different (sand) rectifiers applied to a couple of amplifiers (a SET and a sand), I still can't believe how _MUCH_ these can affect the overall sound, even when they are followed by a ( by all means! ) massive CLC filter (330uF+30H+560uF+470uF+47uF) in one case or when they are used in a hi-NFB SS design which is supposed to (does!) have a huge PSRR.


BTW: now that the idea should be clearer, are there any ideas/suggestion for possible implementation, anyone?


P.S.: Thanks for the drawing.
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Old 12th January 2006, 05:06 AM   #10
Eva is offline Eva  Spain
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peranders:

This switch IC trick is nice. However, what I was trying to point out is that a MOSFET connected to a transformer as a synchronous rectifier has to be turned off just before the input voltage tries to fall below the output voltage. This forces the body diode to conduct for a brief period of time and to show its reverse recovery characteristic, thus there is no EMI improvement over a normal diode (although there is an obvious dissipation improvement).

I'm employing synchronous rectification in my 15V 120A PSU so I know this behaviour quite well. At full load I'm turning my MOSFETs off approx. 500ns before they are asked to stop conducting and I'm turning them back on approx. 800ns after they are asked to start conducting back. Body diode behaviour is crucial during these glitches (particularly due to reverse recovery) altough additional conduction losses are negligible considering that the switching period lasts approx. 16us.
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